Conventional process surveillance schemes are sensitive only to gross changes in the mean value of a process signal or to large steps or spikes that exceed some threshold limit value. These conventional methods suffer from either a large number of false alarms (if thresholds are set too close to normal operating levels) or from a large number of missed (or delayed) alarms (if the thresholds are set too expansively). Moreover, most conventional methods cannot perceive the onset of a process disturbance or sensor signal error that gives rise to a signal below the threshold level or an alarm condition. Most conventional methods also do not account for the relationship between a measurement by one sensor relative to another sensor. Further, most conventional methods provide no means to assess the most likely cause of a process disturbance or sensor signal error. For example, a process disturbance could result from any combination of an instrumentation problem, an equipment problem, or the process operating in a new or unexpected way.
Recently, improved methods for process surveillance have developed from the application of certain aspects of artificial intelligence technology. Specifically, parameter estimation methods have been developed using either statistical, mathematical or neural network techniques to learn a model of the normal patterns present in a system of process signals. After learning these patterns, the learned model is used as a parameter estimator to create one or more virtual signals given a new observation of the actual process signals. Further, high sensitivity surveillance methods have been developed for detecting process and signal faults by analysis of a mathematical comparison between the actual process signal and its virtual signal counterpart. Moreover, automated decision making methods have been developed for reasoning about the cause of events or problems on the basis of their symptoms as represented in data.
Parameter estimation based surveillance schemes have been shown to provide improved surveillance relative to conventional schemes for a wide variety of assets including industrial, utility, business, medical, transportation, financial, and biological systems. However, parameter estimation based surveillance schemes have in general shown limited success when applied to complex processes. Applicant recognizes and believes that this is because the parameter estimation model for a complex process must characterize the entire operating state space of the process to provide effective surveillance. Moreover, a review of the known prior-art discloses that virtually all such systems developed to date utilize a single model of the process to span the entire set of possible operating modes. Hence, a significant shortcoming of the known prior-art is that, inter alia, statistically derived models become extremely large and neural network models become difficult or impractical to train when the process operating state space is complex. The implication for statistically derived models is that the parameter estimation method and system becomes computationally expensive to operate thereby limiting the utility of the method for on-line or real-time surveillance. An alternative for statistically derived models is to constrain the size of the model; however this constraint limits the accuracy of the parameter estimation method and thereby limits the sensitivity of the surveillance method. The implication for mathematical and neural network models is simply that the parameter estimation method and system becomes less accurate thereby degrading the sensitivity of the surveillance method. These shortcomings in parameter estimation and the dependent capability for fault detection also reduce the utility, performance and benefit of automated decision making methods. Further, automated decision making itself becomes a much more complex and less reliable procedure when the process operating state space is complex. Automated decision making when the process operating state space is complex often leads to conflicting and incompatible decision objectives and fault patterns when considering multiple operating modes of the process. In fact, automated decision making when the process operating state space is complex can become combinatorially infeasible to accomplish with the reliability and confidence needed for practical use.
Many attempts to apply parameter estimation, fault detection, and fault classification techniques to assets such as industrial, utility, business, medical, transportation, financial, and biological processes have met with poor results in part because the techniques used were expected to characterize the entire operating state space of the process. In one example, a multivariate state estimation technique (MSET) based surveillance system for the Space Shuttle Main Engine's telemetry data was found to produce numerous false alarms when the learned MSET parameter estimation model was constrained to a size suitable for on-line, real-time surveillance. In this case, the surveillance system false alarm rate could be reduced by desensitizing the surveillance threshold parameters; however, the missed alarm rates then became too high for practical use in the telemetry data monitoring application. In another case, a Bayesian belief network fault classification system for the X-33 Single Stage to Orbit Demonstrator vehicle was found to classify fault indications incorrectly when multiple operating modes of the system were represented in a single decision model.
Moreover, current parameter estimation, fault detection, and fault classification techniques for surveillance of assets such as industrial, utility, business, medical, transportation, financial, and biological processes fail to recognize the surveillance performance limitations that occur when it becomes necessary to trade-off decision processing speed against decision accuracy. This may be attributed, in part, to the relative immaturity of the field of artificial intelligence and computer-assisted surveillance with regard to real-world process control applications. Additionally, a general failure to recognize the specific limitations of trading off decision processing speed against decision accuracy for computer-assisted surveillance is punctuated by an apparent lack of known prior art teachings that address potential methods to overcome this limitation. In general, the known prior-art teaches computer-assisted surveillance solutions that are either applied globally to all operating modes of an asset or applied only to a single predominant operating mode, for example, applied only to steady state operations while neglecting all transient operating states of the asset.
For the foregoing reasons, there is a need for a surveillance system and method that overcomes the significant shortcoming of the known prior-art as delineated hereinabove.